This invention relates to the multi-staged rapid charging of alkaline Nickel-Zinc cells and batteries.
The performance of rechargeable zinc electrodes in alkaline electrolytes has been the subject of many studies that encompass the zinc electrode composition and the interaction with the electrolyte. A performance inhibiting disfigurement of the zinc electrodexe2x80x94shape changexe2x80x94occurs as cycling progresses. The most obvious effect is a lower than acceptable amp-hour capacity delivered at useable voltages. This tendency has been arrested by a number of approaches, particularly as they relate to the composition of the zinc electrode, or the consituency of a nickel-zinc cell.
The combination of more dilute potassium hydroxide electrolyte together with the addition of calcium hydroxide to the zinc electrode appears to be somewhat effective (U.S. Pat. No. 4,358,517). Alternative approaches that have used buffered electrolytes, with and without fluoride additions, have also resulted in increased zinc electrode life span. Noteworthy among these approaches is that described in U.S. Pat. No. 5,453,336 which teaches that a mixture of alkaline electrolyte (2-12M) combined with a carbonate of 0.5-4M and a fluoride of 0.5-4M is beneficial. In U.S. Pat. No. 4,273,841, Carlson describes another mixture that employs 5-10% hydroxide, 10-20% phosphate and 515% fluoride. Eisenberg describes two electrolyte formulations in U.S. Pat. Nos. 4,224,391 and 5,215,836. Both employ mixtures of potassium hydroxide and boric, phosphoric or arsenic acid. However, the latter patent describes advantages of alkali fluorides in the range of 0.01 to 1M. This allows the use of a more alkaline electrolytexe2x80x94an electrolyte having greater alkalinityxe2x80x94with beneficial effects upon the utilization of the nickel electrode material.
Despite the plethora of literature claiming the merit of various configurations of the nickel-zinc system, there appears to be little commercial evidence of the success. As evidenced above, many formulations are credited with increasing the cycle life of the zinc electrode, but a number of problems clearly remain. Principal among these is the loss in capacity of both the zinc and nickel electrodes as cycling continues. A fundamental problem for a nickel-zinc system is the disparity in efficiencies of the zinc and nickel charging process. The need for significant (10% or more) nickel overcharging has frequently been quoted as a problem that results in the overcharge of the zinc electrode and the consequent evolution of hydrogen. A solution to this problem appears to be the optimization of sealed cells that rely on the oxygen recombination cycle. Theoretically, if the recombination is efficient, the charge efficiencies of the electrodes will equilibrate. Unfortunately the recombination efficiency of the zinc electrode is difficult to maintain, and does not approach the levels achieved by nickel cadmium cells. This eventually leads to gas expulsion, dry out of the cell, and cell degradation. Other problems of the zinc electrode are the degeneration of the structure of the electrode, and the gradual passivation of the active material. More directly life threatening to the cell is the formation of separator penetrating dendrites that short the cell, as well as the formation of a mossy variety of zinc that appears to accumulate during cycling.
It is clear that the charging of nickel-zinc cells and batteries has unique requirements if all of these conditions are to be avoided. There are numerous charging schemes directed toward improvement of the zinc cycle life, however no scheme appears to address all of the problems of the zinc electrode. Pulse charging has been claimed to help capacity maintenance in a number of cases. Katz (Journal of Power Sources, 22,77, 1988) determined that 15.7 mA/cm2 at 30 ms on and 90 ms off helped capacity stability over 125 cycles. Binder and Kordesh (Electrochimica Acta 31,255,1986) claimed the benefits of a complex waveform consisting of charge, discharge and a rest period; however, the charge time was longer than for conventional constant current methods. U.S. Pat. No. 4,503,378 describes a constant current method of charging nickel zinc cells. Termination of charge is triggered by the detection of an inflection point in the voltage time curve.
A number of pulse techniques have been successfully used on less sensitive battery chemistries. U.S. Pat. No. 4,829,225 outlines a pulse method that defines a charge pulse followed immediately by an equal or larger discharge pulse. As the charge nears completion, the level or duration of the charge and discharge may be reduced. Another pulse method taught in U.S. Pat. No. 3,517,293 teaches that the frequency of the discharge pulse increases as the charge progresses. Yet another technique taught in JP 8317574A uses current pulses that are lowered prior to gas generation, together with extended off periods.
A multiple stage charger for nickel-cadmium cells is described in U.S. Pat. No. 4,670,703 in which there is a high charge rate, a lower current rate and a trickle charge for capacity maintenance. A similar 3-stage system is described in U.S. Pat. No. 4,952,861 for a lead system, but the total charge time is 5-8 hours and both voltage and time trip points are used.